salt.proxy.fx2: dictionary changed size during iteration

[asenci]

Description of Issue/Question

The "chconfig" function inside the salt.proxy.fx2 module causes a Python runtime error trying to modify the kwargs dictionary while iterating over it:

The minion function caused an exception: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/salt/minion.py", line 1606, in _thread_return
    return_data = minion_instance.executors[fname](opts, data, func, args, kwargs)
  File "/usr/lib/python2.7/dist-packages/salt/executors/direct_call.py", line 12, in execute
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/salt/modules/chassis.py", line 52, in cmd
    return __proxy__[proxycmd](cmd, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/salt/proxy/fx2.py", line 331, in chconfig
    for k in kwargs:
RuntimeError: dictionary changed size during iteration

Looks like the same situation as issue #46765.

I haven't tested, but I'd expect the following modules to have the same behaviour:

salt.modules.nxos, line 62:

    for k in kwargs:
        if k.startswith('__pub_'):
            kwargs.pop(k)

salt.modules.cisconso, line 152:

    for k in kwargs:
        if k.startswith('__pub_'):
            kwargs.pop(k)

salt.proxy.esxi, line 493:

    for k in kwargs:
        if k.startswith('__pub_'):
            kwargs.pop(k)

Setup

Steps to Reproduce Issue

Run "chassis.cmd" with any parameters against a FX2 proxy minion. E.g.:
sudo salt 'dellchassis01' chassis.cmd inventory -t 300 -l debug

Versions Report

Salt Version:
Salt: 2018.3.2

Dependency Versions:
cffi: Not Installed
cherrypy: Not Installed
dateutil: 2.6.0
docker-py: Not Installed
gitdb: 2.0.2
gitpython: 2.1.5
ioflo: Not Installed
Jinja2: 2.9.6
libgit2: 0.24.6
libnacl: Not Installed
M2Crypto: Not Installed
Mako: 1.0.7
msgpack-pure: Not Installed
msgpack-python: 0.4.8
mysql-python: Not Installed
pycparser: Not Installed
pycrypto: 2.6.1
pycryptodome: Not Installed
pygit2: 0.24.2
Python: 2.7.14 (default, Sep 23 2017, 22:06:14)
python-gnupg: 0.3.9
PyYAML: 3.12
PyZMQ: 16.0.2
RAET: Not Installed
smmap: 2.0.3
timelib: Not Installed
Tornado: 4.5.1
ZMQ: 4.2.1

System Versions:
dist: Ubuntu 17.10 artful
locale: UTF-8
machine: x86_64
release: 4.13.0-43-generic
system: Linux
version: Ubuntu 17.10 artful

[gtmanfred]

Thanks for the pr to fix this!
Daniel